C6 to C15 alcohols are produced in large volumes throughout the world by the hydroformylation of olefins to produce aldehydes, followed by hydrogenation of the aldehydes to produce alcohols. The hydroformylation process is also referred to as the Oxo or oxonation process, and the alcohols may also be referred to as oxo-alcohols. The olefins that are used as feeds for the hydroformylation are generally oligomers of olefins that are obtained from petroleum feedstocks. Various processes may be used to produce the olefins used for hydroformylation. For example, the octenes that are used in the production of nonyl alcohol, which is produced in large volumes for the manufacture of plasticiser ester, may be produced by the dimerisation of butenes employing a nickel containing catalyst, e.g., by the Octo® or Dimersol® processes, or dimerisation on a zeolite or other acidic catalyst. These are processes which yield substantially pure octenes. Alternatively olefin mixtures averaging about eight carbon atoms may be obtained by the oligomerisation of olefin mixtures using acid catalysts such as phosphoric acid catalysts.
In both these processes, due to the petroleum origin of the olefins, the olefins typically contain impurities such as sulphur and chlorine which can have a damaging effect on the hydroformylation reaction and, in particular, the hydrogenation reactions. The hydrogenation reactions are performed by catalytic hydrogenation at elevated temperature and pressure and the conditions must be carefully controlled in order to optimise the yield and selectivity of the hydrogenation, ensure safe operation of the hydrogenation unit, secure commercially viable catalyst life and minimize side reactions.
Alternative processes for producing alcohols may comprise the hydroformylation of lower carbon number olefins, such as ethylene, propylene and butenes to the corresponding aldehyde or aldehyde mixtures containing one more carbon number than the starting olefin or olefins. These aldehydes, or mixtures thereof, are then subjected to aldolisation to produce condensation products, typically higher aldehydes containing an extra carbon-carbon double bond, often referred to as enals. These enals or enal mixtures may be hydrogenated to the corresponding saturated aldehydes or aldehyde mixtures, or directly to the corresponding alcohols or alcohol mixtures. Examples of products produced by such processes are 2-methylpentanol, 2-ethylhexanol, 2,4-dimethylheptanol and 2-propylheptanol, but other alcohols and alcohol mixtures produced in this way are also known.
Commercial C6-C15 alcohol mixtures may be produced from C5-C14 olefin mixtures by a process involving hydroformylation followed by hydrogenation. Hydroformylation typically uses a homogeneous metal catalyst, typically rhodium and/or cobalt, often in the carbonyl form. The removal of the metal catalyst may involve oxidizing the metal to a water soluble metal salt. The oxidation may use air as the oxidant.
The subsequent hydrogenation step typically uses a heterogeneous catalyst. Many hydrogenation catalysts are sensitive to poisoning, in particular by sulphur, and this often at relatively low levels. If the olefin feed mixtures contain traces of sulphur, it may be preferred to remove the sulphur before the hydroformylation step in order to protect a sulphur sensitive hydrogenation catalyst downstream. This step may be omitted if the selected hydrogenation catalyst is resistant to sulphur.
During the hydroformylation, the aldehydes that are formed by the main reaction may further react, with H2 to form the alcohol that is often the prime product, with CO and H2 to form a formate ester, and with two alcohol molecules present to form an acetal and water. Water is therefore typically added to the hydroformylation reaction, to promote hydrolysis of formate esters to form an alcohol and formic acid, and/or to push back the acetal formation reaction. In several of the known hydroformylation catalyst cycles, the formic acid byproduct from the hydroformylation step is a useful component. Part of the formic acid byproduct may also remain in the hydroformylation product.
The product from the hydroformylation step, typically after removal of the metal catalyst, may be routed directly to the subsequent hydrogenation step, or unreacted olefins may first be distilled away and optionally recycled, and the remainder of the stream may then be fed to hydrogenation, typically including formate esters, acetals and other heavies.
Also in the hydrogenation step, water is typically introduced, with the purpose to further promote the reduction of formate ester and/or acetals.
Formate esters may thus also during hydrogenation be hydrolysed and produce byproduct formic acid. Some hydrogenation catalysts are more resistant to the presence of formic acid as compared to others. Methanol may be formed as a byproduct from some of the reactions wherein formate esters are reacted away in the hydrogenation step. We have found that the formation of methanol during hydrogenation may depend on the type of hydrogenation catalyst, on the amount of water present, and on the hydrogenation conditions.
A reduction of the acetals in the hydrogenation step down to low levels is particularly important because any acetals left in the hydrogenation product end up as heavies in the bottom byproduct from the alcohol distillation step which typically follows downstream. Some hydrogenation catalysts are better in the removal of acetals as compared to others.
The selection of a hydrogenation catalyst may thus be directed by several criteria in addition to its activity in aldehyde hydrogenation. For example, a sulphided bimetallic catalyst in the hydrogenation step of the alcohol process has good activity in converting formate esters and reducing acetals to very low levels, while withstanding formic acid and sulphur impurities in the hydrogenation feed.
WO 2005/058782 discloses an alcohol process including the hydrogenation of a feed stream having a cold sap number of 38.5 mg KOH/g. The process uses staged water injection in order to improve the reduction of formate esters and of acetals during hydrogenation. WO 2005/058782 proposes to use sulphided bimetallic catalysts because they are not poisoned by sulphur, and proposes to reduce the acidity of the catalyst support in order to avoid excessive by-product formation. WO 2005/058782 is silent about any presence or formation of acids, methyl esters or heavy esters prior to or during the hydrogenation step.
U.S. Publication No. 2005/0065384 discloses an alcohol process including the hydrogenation of a commercially-obtained crude linear nonanal as the feed over supported bimetallic catalysts in their reduced and in their sulphided form. The hydrogenation feeds contain at most 4.5 wt % of formate esters of C9 alcohols, which corresponds to a net cold sap number of 14.68 mg KOH/g. U.S. Publication No. 2005/0065384 is silent about the formation of methanol and methyl esters during the hydrogenation step.
Where aldehydes are present, their corresponding carboxylic acids may be formed via various chemical pathways, such as by reaction with oxygen or by the reaction of two aldehydes with water to form an alcohol plus an acid. Carboxylic acid formation may therefore occur during the different steps of the alcohol process, such as during the hydroformylation, the hydroformylation catalyst removal, and the hydrogenation step. We have found that a sulphided bimetallic hydrogenation catalyst may lead to a higher carboxylic acid formation as compared to others.
These carboxylic acids are typically undesired in the alcohol production process. Their hydrogenation to alcohol is relatively difficult and slow. Having the same carbon number of the alcohol prime product, the acids are less volatile but remain relatively difficult to separate from the prime alcohol product. When distilled away, their downgrade into the heavy byproduct stream represent a loss of useful molecules. Because the separation by distillation is difficult, the acid containing heavy byproduct may also contain some alcohol, representing further loss of useful molecules.
We have also found that hydrogenation catalysts such as the sulphided bimetallic catalysts may enable esterification. The carboxylic acids may then react in the hydrogenation step with alcohol to form a heavy di-alkyl ester having twice the carbon number of the alcohol, and water. These heavy esters remain in the bottom of the alcohol distillation and represent an even more important loss of useful molecules as compared to the acid alone.
As methanol may be present during hydrogenation, the esterification of the carboxylic acid may also lead to the formation of methyl esters.
We have found that the methyl esters of the carboxylic acids having the same carbon number of the aldehydes, and thus also of the product alcohol, do not separate from the alcohol product mixture by distillation, and thus remain primarily as an impurity in the product alcohol mixture. We have also found that it is very difficult to analyse for such methyl esters in a C6-C15 alcohol mixture.
The methyl ester impurity in the alcohol product may cause problems when the alcohol product is further esterified to an ester derivative. A particular problem occurs when an ester derivative of a polycarboxylic acid or its anhydride is produced, such as a phthalate, an adipate, a trimellitate or a cyclohexane dioate ester. Several of these esters of polycarboxylic acids are used as plasticizers for a polymer, typically for polyvinyl chloride (PVC). We have found that during esterification, the methyl ester may be subject to transesterification, freeing up the methanol. The methanol moiety in the methyl ester may be replaced by a parent alcohol moiety and a di-alkyl ester is then formed which has two alkyl chains with typically the same carbon number as the parent alcohol that was used for the esterification. We have also found that when an ester of a polycarboxylic acid is produced, most of the aliphatic di-alkyl esters formed during esterification remain as an impurity in the product ester. This impurity is undesirable because it may increase the volatility of the ester product, and it may contribute negatively to its performance, such as to the reading in the fogging test, or to the light scattering film performance, of the plasticiser ester or of an article derived there from. This is particularly important for the automotive industry, but also for other applications such as when articles are produced for indoor use.
The methanol that is liberated by the transesterification of the methyl ester may end up in the water byproduct from the esterification process, where it represents an environmental burden in the disposal of the waste water. The methanol may also react with the acid or acid anhydride used in esterification, such as adipic acid or trimellitic or phthalic anhydride, to produce undesirable di-methyl phthalates or adipates, or equivalent di-esters with one methyl and one C6-C15 alkyl group, or a trimellitate with one or two methyl groups instead of the desired alkyl group with more carbon atoms.
The di-alkyl aliphatic esters that are produced can be carried with the desired ester during its purification and can remain as an impurity in the final ester product and large quantities can render the ester unsuitable for use as a plasticiser. Even in smaller quantities, they can impair the volatility of the plasticiser and affect the fogging performance of the plasticiser of the finished article produced therewith. The presence of these undesired esters can be detected and quantified by a GC analysis of the product ester. With phthalate esters for instance, these di-alkyl aliphatic (mono-)esters elute in the region that is called “Intermediates”, i.e., the region of the phthalate ester GC spectrum where the “dimer” impurities elute. Also the methyl-containing tri- or di-esters will tend to elute typically before the peak of the main tri- or di-ester, because of their lower molecular weight.
There therefore remains the need for a process for producing an alcohol wherein the formation of carboxylic acids having the same carbon number as the alcohol is reduced, in particular during the hydrogenation step and with a sulphided bimetallic hydrogenation catalyst which is more active in acetal reduction, which has a better formic acid resistance or a higher resistance to sulphur poisoning. Preferably, also the esterification of such carboxylic acids is reduced. A further need remains for a process to produce an alcohol wherein the formation of the methyl esters of such carboxylic acids during the hydrogenation step is reduced, because of the product quality problems these methyl esters may create. There remains also a need for an alcohol that contains only a limited amount of such methyl esters. The downstream process producing an ester derivative from such an alcohol with a polycarboxylic acid, is in need for an alcohol that contains only low amounts of methyl ester, such that the environmental burden associated with disposal of its byproduct water is reduced, and also such that its ester product contains lower amounts of di-alkyl esters and/or di- or tri-esters having one or more methanol moieties, leading to a product of higher purity which provides improved performance during downstream processing and improved performance of the derived consumer product during its useful lifetime.